This invention related to momentum transfer actuators (MTAs), and, in particular, to the use of momentum transfer actuators to control the motion of a flexible mechanical structure.
It is well known in the art to actively damp the vibrations caused by a machine so as to prevent the coupling thereof to the surrounding environment. It is also well known in the art to actively damp unknown vibrations from the environment so that they do not disturb a motion sensitive piece of equipment, such as a camera or photolithography machine.
Typically, the application of inertial force within a support member, under control of a feedback system, is used to cancel out the force applied to the vibrating side of the support member to achieve vibration isolation between the vibrating side of the support member and the xe2x80x9cquietxe2x80x9d side of the support member. The feedback system senses forces applied to the member and cancels the forces by the application of equal and opposite forces. This concept is the basis for a wide variety of active vibration damping schemes, such as those disclosed in U.S. Pat. No. 4,483,425 (Newman), U.S. Pat. No. 4,525,659 (Imahashi, et al.), U.S. Pat. No. 4,929,874 (Mizuno, et al.), U.S. Pat. No. 5,156,370 (Silcox, et al.), U.S. Pat. No. 5,327,061 (Gullapalli) and U.S. Pat. No. 5,750,897 (Kato). These prior art inventions all make use of an additional movable mass that is accelerated in such a way as to cancel out the inertial forces that would otherwise be coupled into a base plate through a coupling member. Examples of both linear motion and rotational motion of these inertial proof masses are disclosed in the prior art.
In many cases improved vibration damping of a mechanical structure is achieved by simply adding more material to increase the stiffness of the structure. In such a case, a given disturbance force will result in a smaller motion at the flexible end of the mechanical structure. However, there are a many economically important applications in which other design constraints limit the degree to which increased material can be added to the system. For example, the mechanical beams used to construct a space station must be extremely light because they have to be lifted into space from Earth""s gravity well. When the mass or volume of the mechanical system is constrained, the flexibility of mechanical components can often limit the performance of the overall system.
In a typical hard disk drive there is a mechanical arm that holds the read/write head at a precise radial position over a desired track of data (see FIG. 1). This arm is attached by a rotary bearing at a point just outside the outer circumference of the disk. A voice coil actuator is attached to an extension of this arm on the opposite side of the bearing as the read/write head. Force from the voice coil actuator moves the back end of the arm, thereby causing the arm to pivot (see FIG. 2). The maximum force that can be generated by the voice coil actuator is limited. Therefore, to achieve a required arm seek time, the rotational moment of inertia of the arm, and similarly its total mass, must be limited. For example, note the holes intentionally cut into the arm example shown in FIG. 3 to reduce its mass. Because only a limited amount of material (mass) can be used in the manufacture of the arm, the arm exhibits an undesired degree of flexibility, and cannot be made arbitrarily stiffer by simply adding more material.
There are three main disturbances in a hard disk drive that affect the position of the read/write head relative to the desired data track on the diskxe2x80x94actuator bearing hysteresis, windage disturbance, and non-repeatable disk bearing noise (a.k.a. non-repeatable runout). Actuator bearing hysteresis is a time-dependent nonlinear behavior typical of mechanical bearings. It typically results in an initial delay in motion resulting from a force applied to the voice coil actuator followed by some overshoot in the desired motion. The windage disturbance is a non-predictable turbulence force that is applied to the head. Non-repeatable bearing noise results in the disk itself moving slightly over time in an unpredictable way. Note this unpredictable disk motion results in a relative position error of the head since it must be over a specific track on the disk for read or write operations to be successfully executed.
Because of the above disturbances, the read/write head may not always be able to be positioned over the precise spot on the magnetic disk surface that is required for a successful read or write. Therefore, it is desirable that the portion of the arm to which the read/write head is attached be able to be positioned over the correct spot on the disk despite the disruptive forces described above. In general, it is desirable that a flexible mechanical structure, such as the arm of a hard disk drive, be made to follow a precise mechanical position or track a precise mechanical path (e.g., track out the non-repeatable runout of the disk) as a function of time, in the presence of disruptive external forces (e.g., the vibration or high frequency oscillation caused by windage forces).
This invention, like the prior art described above, makes use of inertial force as generated by an (MTA) to affect the motion of a flexible support member. However, the current invention addresses a very different application for this same basic device. Instead of attenuating or blocking the transmission of a force or torque through a flexible structural member, the focus of this invention is the use of feedback controlled inertial force, added to the existing force transmitted in the flexible structural member, to precisely control the position of a specific part of a flexible mechanical structure as a function of time. This invention applies to the general case in which it is desired that the position of some part of a flexible mechanical structure follow a precise function of time, and one or more MTAs are used to both cancel disturbance forces and to position the flexible mechanical structure to follow a desired trajectory.
Specifically, the invention related to the head of a hard disk drive, which is typically mounted at the end of a flexible mechanical structure. In a typical hard disk drive, the position of the read/write head relative the desired data track is determined periodically whenever the read/write head passes over sector servo marks which are embedded between data tracks. A typical servo burst rate in today""s hard disk drives is on the order of 10 KHz. For feedback loops operating at high frequencies, the phase delay caused by this periodic sampling of the position error signal may result in poor stability. Although this could be remedied by simply using more frequent servo bursts, doing so would take away surface area that could be used to store data. Therefore, is preferred to make use of an inertial sensor to augment the sampled data position signal available from the servo bursts. One possible use of this system would be to interpolate between samples using the inertial sensor.
Another aspect of this invention is the use of an inertial sensor combined with an MTA to provide acceleration, velocity, and position information concerning the location of the mechanical structure to which the MTA is attached. This information is used in addition to other acceleration, velocity, or position signals in order to improve the performance of the overall feedback system.